In the prior art techniques, a color image forming device utilizes a tandem method of successively transferring an image that has been formed by the electrophotographic process onto transferring paper on a transfer belt. A plurality of image forming units includes a cyan image forming unit, a magenta image forming unit, a yellow image forming unit and a black image forming unit, and each of the image forming units is equipped with a writing sub-unit and an image forming sub-unit for forming a static image on a photoreceptor drum by a modulated laser according to the corresponding image data. Subsequently, each of the image forming units supplies corresponding color toner onto the photoreceptor according to the static image. Finally, the formed toner image is successively transferred onto an image-forming medium on the transfer belt in an overlapping manner so as to form a full color image.
In general, the misalignment is grouped either angled misalignment or parallel misalignments. The angled misalignments are caused by an erroneous positioning of the optical system, the image forming units and the photoreceptor drum with respect to the color image forming device body. The angled misalignments are generally corrected by adjusting the position of a reflective mirror in the writing unit. The parallel misalignments are caused by an erroneous positioning of the main-running direction with respect to a predetermined standard line. The parallel misalignments are generally corrected by adjusting the writing timing of the main-running direction or the sub-running direction. The length of the image along the main-running direction is also adjusted by the frequency of the writing pixels to correct a scaling error.
In the above tandem method, color misalignments also occur for various reasons. In comparison to a single drum method, the tandem method requires the successive and overlapping color toner transfer onto the same image forming medium. Although the tandem method operates at a high printing speed, it is difficult to align the colors. For example, when a user or a repairman accidentally moves a part of the electrophotographic components from the predetermined position in response to a paper jam, even if the moved component is put back to a supposedly original position, a minute positional difference causes a color misalignment. When the color toner fails to transfer at an exactly predetermined position from each of the image forming units, the formed full color image contains undesirable color overlapped portions which are not in intended colors. The color may undesirably appear faded or darkened in these portions. In other situations, the portions contain undesired gaps or overlaps.
In order to correct the color positional misalignment in the above described color image forming device, the printing task is interrupted, and position detection patterns are formed on the transfer belt so that CCD sensors detect the position detection patterns in order to determine an amount of the color positional misalignment for the correction. The position detection patterns vary in density and uniformity due to the environmental changes such as temperature and humidity and other changes over time. As a result, the position detection patterns are not correctly formed as desired. When the position detection patterns are not correctly formed, since the position detection patterns are not correctly detected, the color positional misalignment is not corrected in a precise manner.
In attempt to solve the above problem, another prior art such as disclosed in Japanese Patent Publications Hei 10-260567, Hei 7-181795 and 2002-14505 disclose a corrective method in which the position detection patterns are detected and the detected density results are compared to a predetermined range of values. If the detected density results are out of the predetermined range, the density-adjusted position detection patterns are again formed for an accurate correction in the color positional misalignment process. Unfortunately, the above prior art technique requires an additional amount of time at the detection for the color positional misalignment correction since the density is always determined for a density adjustment process during the color misalignment correction process.